The present invention relates to a soft program method in a non-volatile memory device for performing a soft program step so as to improve threshold voltage distribution of an erased cell.
Recently, the demand has increased for non-volatile memory device which electrically programs and erases data, and does not require a refresh function for periodically rewriting data.
The non-volatile memory device generally includes a memory cell array having matrix-typed cells for storing data, and a page buffer for programming data to a specific cell in the memory cell or reading data from a cell.
The page buffer has a pair of bit lines connected to a given memory cell, a register for temporarily storing data to be programmed to the memory cell or data read from a cell, a sensing node for sensing a voltage level of a specific bit line or voltage level of a certain register, and a bit line selecting circuit for controlling connection of the bit line and the sensing node.
When an erase operation is performed in the non-volatile memory device, a soft program operation is performed so as to improve threshold voltage distribution of an erased memory cell (e.g., when a cell is over erased). However, in a conventional soft program operation, though only a threshold voltage of one cell included in one cell string is higher than a verifying voltage, a soft program operation for the whole block is stopped. Accordingly, the threshold voltage distribution of the erased cells may become wide with the use of the soft program operation.